In health care, it is often desirable to monitor respiration of a human being (or correspondingly, of an animal in veterinary medicine). For example, respiration is affected during anesthesia, since the respiratory centre of the brain is then brought is then brought out of function. Usually, the patient is mechanically ventilated during anesthesia, and is brought back to normal respiration during the wakening procedure. In this state of awakening, there is a risk of apnea (break arrest), which can be very traumatic and risky to the patient.
Further, it is known that the respiratory function of newborn babies sometimes is not fully developed, which can also lead to apnea, and in some cases to death (the so called sudden infant death syndrome). The direct causes of these tragic cases are still not sufficiently known, but there seems to be a consensus that apnea is the primary symptom, and that this may be easily heaved if it is discovered early enough.
From the cases mentioned above, it should be clear that monitoring respiration is highly important in certain groups of patients. The technological problem of detecting this function has, indeed, been he subject of several innovations and designs. One can classify these detectors into, on the one hand, those which record the respiratory motion of the patient, and on the other those which detect the flow of air resulting from respiration. The first kind could consist of a belt tied around the chest, with a sensor detecting the variations of length or tension caused by respiration motion. Alternatively, the so called transthoracid impedance is recorded with electrodes placed on the chest wall. Since the impedance is varying when the lungs are filled with air, or emptied, a signal is received which can be used for respiration monitoring.
Common to these methods of recording chest motion is, however, that they are sensitive also to other movements of the patient. This is especially serious, since it can give rise to a false negative response, i.e., the monitoring equipment records an artefact as the desired signal and does not give an alarm despite a possible breath arrest of the patient.
A detector of the flow of expired air does not have this serious limitation, since the detection principle is not coupled to the motion of the patient, given that the detector is fixed to the patient's mouth and nose in such a way that it is hit by the stream of air independently of the motion of the patient, and that the detector is protected from other sources of air flow, e.g., wind or draught.
Gas flow can be detected in several ways. The technical literature is dominated by variants of hot wire anemometer which is based on detection of the temperature change caused by the cooling effect of streaming gas on a heated body. For respiration detection one can make use of the fact that expired air is heated by the human body, making it very simple to place a sensitive temperature sensor in the air flow. This principle has, together with impedance measurements, become the dominant detection principle in clinical practice. It is, however, hampered by several drawbacks. For example, a fast response is required in order to to record single breaths. Therefore, only miniaturised temperature sensors, e.g. thermistors with extremely small mass, can be used. These are difficult to handle, since the very small connecting wires easily break. To guarantee patient safety against electric shocks, the sensor connections must be galvanically isolated from all other equipment, which requires expensive isolation amplifiers. Furthermore, drops of condensed water can develop on the sensor due to the high concentration of water vapor of expired air. Then the response of the sensor is altered, and its function as a respiration detector may be lost.